(1) Field of the Invention
The present invention relates to techniques for compensating for influences on signal light due to polarization-mode dispersion of transmission lines. In particular, the invention relates to a method and apparatus for compensation of polarization-mode dispersion, which detects the strength of a specific frequency component in a baseband spectrum in signal light, to perform compensation of polarization-mode dispersion.
(2) Related Art
Presently, in Japan and overseas, the commercialization of optical transmission systems at transmission speeds of 10 Gb/s (gigabit/second) is advancing. Furthermore, in order to provide communication services of low cost and with high frequency efficiency to meet the rapid increase in transmission capacity demand due to the Internet and the like, there is a requirement to realize ultra high-speed optical transmission systems at transmission speeds of for example 40 Gb/s.
However, in such ultra high-speed optical transmission systems, the transmission waveform of the signal light deteriorates due to the influence of polarization-mode dispersion (referred to hereunder as PMD) and the like which occurs in the transmission line. Therefore, there is a problem that the transmission distance of the signal light is limited. This PMD is phenomenon that occurs due to a difference in propagation delay times of polarization components in the signal light (for example two mode light such as TE mode and TM mode), being inevitable phenomena for all optical fibers.
Consequently, in order to realize long distance optical transmission at ultra high-speed, application of PMD compensation technique is essential. Furthermore, since PMD also fluctuates with time due to changes in the transmission line environment, such as temperature or stress, automatic PMD compensation techniques are necessary for monitoring the condition of the PMD during system operation, and performing feedback control.
As a heretofore automatic PMD compensation technique, there is reported for example a compensation method in optical regions (refer, for example, to T. Takahashi et al., Electronics Letters Vol. 30, pp. 348-349, 1994, or F. Heismann et al., ECOC ""98 Technical Digest, pp. 529-530, or Japanese Unexamined Patent Publication No. 11-196046) and a compensation method in electrical stages (refer, for example, to H. Bxc3xclow, NOC ""97 Technical Digest, pp. 57-72).
Furthermore, the present inventors have, from the view point of simple configuration, and independence from modulation methods or other waveform deterioration factors (wavelength dispersion, non-linear effects), and high-speed benefits, proposed an automatic PMD compensation technique which adopts a compensation method in optical regions (refer, for example, to Japanese Patent Application No. 11-515959, or H. Ooi et al., OFC ""99, Technical Digest WE5 pp. 86-88, 1999). This compensation technique is one which adopts a PMD monitor method of a simple configuration not requiring a large scale measuring instrument, to detect the strength of a specific frequency component in a baseband spectrum after transmission (for example, a strength of a 20 GHz component in a 40 Gb/s signal light), and then feedback controls an compensation amount so that the detection strength becomes a relative maximum. By applying this compensation technique, the transmission distance of the signal light is extended by at least four times.
However, in such a compensation technique, there is a problem in that the upper limit of the compensatable PMD is restricted to one time slot of the transmission light. That is to say, as shown in FIG. 12, the strength of the specific frequency component changes with respect to an optical delay amount xcex94xcfx84T (hereunder, PMD amount xcex94xcfx84T) between polarization-modes, due to the PMD in the transmission line, and when the PMD amount xcex94xcfx84T becomes the one time slot of the transmission light (for example 25 ps in the case of 40 Gb/s signal light), the strength of the specific frequency component becomes zero (or relative minimum). Therefore, in the case where the PMD amount xcex94xcfx84T exceeds the one time slot, when the compensation amount is feedback controlled so that the strength of the specific frequency component becomes relative maximum, the PMD amount xcex94xcfx84T after control increases, so that deterioration of the transmission light waveform becomes large.
With respect to this problem, as a technique for extending the range where PMD compensation is possible, a method has been proposed where for example a frequency B/2 Hz component, a frequency B/4 Hz component and a frequency B/8 Hz component in a baseband spectrum in a transmission light signal at transmission speed B b/s are extracted by a band pass filter (BPF), and the various strength are detected (refer, for example, to D. Sandel et al., Electronics Letters Vol. 34, pp. 2258-2259, 1998).
FIG. 13 is a view for explaining the aforementioned PMD compensation technique. The horizontal axis shows the value (xcex94xcfx84T/T) obtained by standardizing the PMD amount xcex94xcfx84T by one time slot T of the transmission light, while the vertical axis shows the strength of the frequency component extracted by each BPF. Here, the curve shown by (BPF 0.5/T) represents the strength of the B/2 Hz component, the curve shown by (BPF 0.25/T) represents the strength of the B/4 Hz component, and the curve shown by (BPF 0.125/T) represents the strength of the B/8 Hz component. Moreover, the curve shown by LPF represents the strength of the B/8 Hz component extracted by a low pass filter (LPF).
As shown in FIG. 13, the lower the frequency extracted by the BPF, the higher the PMD amount xcex94xcfx84T at which the strength of each component becomes zero. Therefore, the range where compensation is possible is extended. However, in the region where the PMD amount xcex94xcfx84T is small, the change in the detection strength (monitor strength) becomes small (each curve approaches a flat). Therefore, in the case where the PMD compensation amount is feedback controlled so that the monitor strength becomes relative maximum, convergence of the feedback control becomes poor. Furthermore, since there is an indefinite width in the sensitivity of the monitor system, there is also the possibility of time-wise unstable control. Therefore, with the aforementioned PMD compensation technique, three PMD monitors for detecting the strength of each frequency component are sequentially switched. With regards to the switching of the PMD monitors, the threshold values Th1, Th2 for the monitor strength are set beforehand. For example, in the case where the monitor strength increases, when the detection value (curve BPF 0.125/T) of the PMD monitor which detects the strength of the B/8 Hz component, increases up to the threshold value Th1 at the top of FIG. 13, the monitor is switched to the PMD monitor for detecting the strength of the B/4 Hz component. After this, when the detection value (the curve BPF 0.25/T) of this PMD monitor increases to the threshold value Th1, the monitor is switched to the PMD monitor for detecting the strength of B/2 Hz component. Furthermore, for example, in the case where the monitor strength reduces, the monitors are sequentially switched based on the threshold value Th2 at the bottom of FIG. 13, from the PMD monitor for detecting the strength of the B/2 Hz component, to the PMD monitor for detecting the strength of the B/4 Hz component, and then to the PMD monitor for detecting the strength of the B/8 Hz component.
In this way, by controlling so that the plurality of PMD monitors are sequentially switched in accordance with the threshold values Th1 and Th2 previously set with respect to the monitor strength, the range where PMD compensation is possible can be increased to one time slot or more.
However, with the abovementioned conventional PMD compensation technique which controls the switching of the plurality of PMD monitors, absolute values are used for the previously set threshold values for switching the PMD monitors. Therefore, there is the problem in that it is difficult to perform PMD compensation at a high accuracy. That is to say, it is known that the strength of each frequency component detected by the respective PMD monitors fluctuates in accordance with for example wavelength dispersion due to temperature fluctuations or the like, or time-wise changes of the parameters of the branching ratio etc. of the optical strength between polarization-modes, other than with PMD of the transmission line. If the threshold value which becomes the reference for the switching control of the PMD monitor, is previously set using the absolute value rather than the relative value, the setting of the threshold value becomes inappropriate with respect to strength changes due to factors other than PMD as mentioned above, and realization of highly accurate PMD compensation is difficult.
Furthermore, with the conventional compensation technique using a plurality of PMD monitors, there is the restriction that the frequency components for strength detection must be specific frequencies such as xc2xd times, xc2xc times, or xe2x85x9 times of the transmission speed B of the signal light. This restriction makes application to a system where the transmission speed of the signal light changes difficult, and also has the disadvantage that, in order to further widen the compensation range, a large number of frequency components must be monitored.
The present invention is directed to the aforementioned points, with the object of providing a method and apparatus for compensation of polarization-mode dispersion, which can compensate for polarization-mode dispersion (PMD) occurring in signal light, at high accuracy over a wide range.
In order to achieve the above object, the PMD compensation method according to the present invention first compensates for PMD occurring in signal light input via a transmission line. Then, a plurality of specific frequency components in a baseband spectrum in the post compensation signal light is extracted, and thereafter, the strength of each of the specific frequency components is respectively detected. After this, a PMD compensation condition is feedback controlled so that the strength of all of the detected frequency components are within a maximum value convergence range determined in accordance with an indefinite width of the detection sensitivity. Furthermore, at the time of feedback controlling the PMD compensation condition, switching of the strength of the specific frequency components used in feedback control of the PMD compensation condition may be sequentially performed so that, after the strength of a specific frequency component on a relatively low frequency side comes within the maximum value convergence range, the strength of a specific frequency component on a relatively high frequency side comes within the maximum value convergence range.
With such a PMD compensation method, by controlling the PMD compensation condition so that the strength of the plurality of specific frequency components converge on the respective maximum values, feedback control accurately following the change in the PMD amount of the signal light becomes possible. Furthermore, by sequentially switching the strength of the specific frequency component used in the feedback control from the low frequency side to the high frequency side, the switching of PMD monitors is performed on the basis of the strength of maximum value convergence condition as the base rather than on the basis of the absolute threshold value as heretofore. Therefore, high accuracy PMD compensation can be realized.
The PMD compensation apparatus according to the present invention for compensating for influences on signal light due to PMD of a transmission line, comprises: a polarization-mode dispersion compensation section for compensating for PMD occurring in signal light input via a transmission line; a specific frequency component extraction section for extracting a plurality of specific frequency components in a baseband spectrum in signal light output from the polarization-mode dispersion compensation section; a strength detection section for respectively detecting the strength of each of the specific frequency components extracted by the specific frequency component extraction section; and a compensation condition control section for feedback controlling a PMD compensation condition in the polarization-mode dispersion compensation section so that the strength of all of the specific frequency components detected by the strength detection section are within a maximum value convergence range determined in accordance with an indefinite width of the detection sensitivity. Furthermore, the aforementioned compensation condition control section may sequentially perform switching of the strength of the specific frequency components used in feedback control of the PMD compensation condition in the polarization-mode dispersion compensation section so that, after the strength of a specific frequency component on a relatively low frequency side comes within the maximum value convergence range, the strength of a specific frequency component on a relatively high frequency side comes within the maximum value convergence range.
With such a construction, in the signal light compensated by the polarization-mode dispersion compensation section, the strength of the plurality of specific frequency components are detected by the specific frequency component extraction section and the strength detection section. In the compensation condition control section, control of the PMD compensation condition in the polarization-mode dispersion compensation section is performed using the detected strength of the respective specific frequency components, so that each of the strength converges on the maximum value. As a result, feedback control accurately following changes in the PMD value of the light signal becomes possible. Furthermore, in the compensation condition control section, by performing control for sequentially switching the strength of the specific frequency component used in the feedback control from the low frequency side to the high frequency side, the switching of PMD monitors is performed on the basis of the strength of the maximum value convergence condition rather than on the basis of the absolute threshold value as heretofore. Therefore, high accuracy PMD compensation can be realized.
Furthermore, the abovementioned PMD compensation apparatus, as a specific construction for the specific frequency component extraction section, may have a band pass filter having a transmission center frequency corresponding to the specific frequency component, to extract the specific frequency component using this band pass filter. Alternatively, this may have a low pass filter having a cutoff frequency corresponding to the specific frequency component, to extract the specific frequency component using this low pass filter.
According to such a specific construction, the plurality of specific frequency components in the baseband spectrum in the signal light, are extracted by the band pass filter or the low pass filter. In particular, in the case where the specific frequency components are extracted using the low pass filter, a value can be obtained for where the strength of the frequency components over a wide range are integrated. Therefore, a more stabilized PMD compensation can be realized.
Furthermore, the polarization-mode dispersion compensation section of the aforementioned PMD compensation apparatus may have a polarization control section for determining a branching ratio for the optical strength to two polarization-modes for the signal light, and an optical delay section for applying an optical delay difference between the two polarization-modes, so that a polarization-mode dispersion compensation amount may be set corresponding to a combination of the branching ratio of the polarization control section and the optical delay difference of the optical delay section.
With such a construction, the polarization condition of the signal light input via the transmission line is adjusted by the polarization control section, so that the branching ratio for the optical strength to the two polarization-modes of the signal light is set, and also the signal light passes through the optical delay section, so that an optical delay difference is applied between the two polarization-modes. Thus, the PMD compensation amount is set to a required value.
Furthermore, as a specific construction of the PMD compensation apparatus, the polarization-mode dispersion compensation section may have a variable optical delay element capable of changing an optical delay difference and a wave plate which changes the polarization direction of the input light to the variable optical delay element, and the compensation condition control device may feedback control at least one of an optical delay amount of the variable optical delay element and a position of the wave plate. Alternatively, the polarization-mode dispersion compensation section may have a plurality of polarization-mode dispersion compensation units connected in series, and each of the polarization-mode dispersion compensation units may contain a fixed optical delay element which applies a previously set optical delay difference, and a wave plate which changes the polarization direction of the input light to the fixed optical delay element, and the compensation amount control section may respectively feedback control a position of the wave plate of each of the respective polarization-mode dispersion compensation units.
The PMD compensation apparatus according to the present invention as described above can be applied to various optical transmission systems. In this case, the polarization-mode dispersion compensation apparatus is provided along a transmission line connecting between an optical sender and an optical receiver. With such an optical transmission system, since the PMD of the signal light propagated through the transmission line is reliably compensated for over a wide range, high-speed signal light can be transmitted over a long distance.
Other objects, features and advantages of the present invention will become apparent from the following description of embodiments given in conjunction with the appended drawings.